The term "lock viewer" goes back to a time when production of a muzzle-loader's lock was a specialized field, which was often subcontracted out. Once built, the locks were brought to government factories to assemble the weapon. The lock viewer's mark, applied by a specialist, (a lock viewer) indicated that the lock had passed inspection.

The lock viewer's mark was applied to service Martini-Henry rifles and carbines to indicate that the weapon's lock, or action has been inspected, and found to function properly, and that it conformed to the specifications of the pattern.

Martini-Henrys dated 1898 and later will not have the lock viewer's mark, as it was deemed to be superfluous in 1897.

The Lock Viewer Marks of the military Martini manufacturers. Note that the BE Co. is not represented, as they did not start production until after the use of the lock viewer's mark was abolished... 1. BSA & Co. 2. Enfield 3. HRB Co. 4. LSA Co. 5. N A & A Co.